1. Field of the Invention
The present invention relates to cooling systems and, more particularly, to cooling systems for electrical apparatus. Generally, the present invention provides a cooling system for apparatus powered by electricity, that generates a substantial amount of heat during operation, and the heat must be dissipated to avoid failure of electrical and/or electronic components, such as semiconductor devices and integrated circuits, comprising the electrical apparatus. Specifically, one embodiment of the present invention provides a cooling system preferably employing liquid ice jet impinged on a heat sink thermally coupled to electrical apparatus, and the attendant phase changes of the liquid ice first to water and then to steam to remove a substantial amount of waste heat to prevent failure of the electrical apparatus.
2. Description of the Prior Art
Cooling is an important process associated with operation of high-density electronic devices. Existing waste heat removal technology is limited to approximately 100 W/cm2.
In the next ten years, the power density of high-power electronics is expected to increase and generate waste heat that will exceed 1,000 W/cm2. Thermal management technology capable of removing waste heat of 1,000 W/cm2 produced by advanced power electronic devices is needed.
For example, the U.S. Department of Navy has reported that the cooling requirements are expected to increase at least an order of magnitude during the next decade. As stated in xe2x80x9cNext Generation Navy Thermal Management Program,xe2x80x9d CARDIVNSWC-TR-82-2002/12, by Michael Kuszewski and Mark Zerby, Naval Surface Warfare Center:
xe2x80x9cIt is expected that heat fluxes for new technologies such as Advanced Radar will exceed 1000 W/cm2, and some advanced weapons may be higher. These heat fluxes are expected to be present by the end of this decade. Heat fluxes are growing so fast in the electronics arena that even Intel, who has been designing its Thermal Management Systems to handle less than 100 W/cm2, has extrapolated its increase of heat flux to reach 1000 W/cm2 before the end of this decade.xe2x80x9d
Accordingly, the U.S. Navy recently published RFQ N03-T022 Acquisition Program: DD(X); CVN(X) having the:
xe2x80x9cOBJECTIVE: To develop advanced thermal management technologies to improve high flux waste heat removal by a factor of 10X over existing technologies in electronic devices.xe2x80x9d
Also,
xe2x80x9cThe proposed solution must be able to keep the semiconductor junction below 125F [sic, 125C]. . . .xe2x80x9d
Considered in more detail, spray cooling with water is a known cooling technique to remove heat from electronics relatively efficiently. See, Kuszewski and Zerby, supra. In situations where cooling very hot surfaces or protecting sensitive surfaces from overheating is important, then the most effective technique available is direct impact by impingement jets (not necessarily sprays). The reverse side of a mounting plate, on which the electronic devices are disposed, is sprayed by high velocity impinging jets of water. The heat generated by the electronics is removed at constant temperature by the liquid vapor phase of the water.
The heat transfer processes involved in water sprays impinging on hot surfaces have been studied by, among others, Bemardin J D, and Mudawar I, xe2x80x9cFilm boiling heat transfer of droplet streams and sprays,xe2x80x9d Intl. J. Heat Mass Transfer, 40 (11), 2579-2593 (1997). Rockwell has also published a paper that reports having achieved removal of 1,000 W/cm2 using a water jet plus boiling. However, Rockwell was only able to cool a very small area (unspecified).
The challenge presented by the need to conduct waste heat from electronic devices efficiently and to provide removal of waste heat on the order of 1,000 W/cm2 at a rate that will maintain the operating temperature of electronic devices at or below 125xc2x0 C. is imposing. The 125xc2x0 C. limit requires efficient heat transfer to sink heat away from the electronic apparatus. The high heat flux (1,000 W/cm2) further requires an effective heat removal process to maintain the operating temperature of electronic devices at or below the 125xc2x0 C. limit.
It would therefore be desirable to provide removal of waste heat from electronic devices to maintain the operating temperature of electronic devices at or below 125xc2x0 C. It would also be desirable to remove waste heat at a rate to prevent the operating temperature of electronic devices from exceeding the 125xc2x0 C. limit. Furthermore, it would be desirable to achieve these objectives for electrical apparatus that generates waste heat on the order of 1,000 W/cm2.
One embodiment of the present invention provides a cooling system for thermally conducting and removing high heat flux waste heat. The cooling system in accordance with one embodiment of the present invention employs a refrigerant or coolant, preferably, liquid ice, and, preferably, at reduced pressure to improve high heat flux waste heat removal by a factor of ten times over known cooling techniques. One embodiment of the cooling system in accordance with the present invention is especially suitable to the challenge of removing high heat flux waste heat resulting from operation of power electronics given the severe limitation on the maximum operating temperature allowable for electronic devices.
One preferred embodiment of the cooling system in accordance with the present invention provides a heat transfer plate consisting of copper, aluminum, silver, or another suitable thermally conductive material, such as beryllium oxide ceramic, boron nitride, aluminum nitride ceramic, or diamond, with high tensile strength to enable efficient heat transfer by thermal conduction, in thermal contact with the electrical apparatus. The heat transfer plate also serves as a structural component of a circulation subsystem that contains the refrigerant or coolant. Impinging jets deliver copious amounts of a refrigerant or coolant to the hot surface of the heat transfer plate opposite the side on which the electronic apparatus is disposed in thermal contact with the heat transfer plate. In a preferred embodiment of the present invention, jet impingement of a refrigerant or coolant in the form of liquid ice is employed.
Jet impingement of liquid ice is provided on the heat transfer plate at atmospheric pressure or at a reduced pressure. Preferably, the liquid ice may be maintained at less than atmospheric pressure, for example, in a partial vacuum, wherein the temperatures associated with phase changes of melting and boiling are lowered.
With the operating temperature of electronic devices required to be at 125xc2x0 C. or below, and the temperature of the liquid ice at approximately xe2x88x922xc2x0 C., there is a large temperature differential and two phase changes as the liquid ice first transforms to water and then to steam to effect heat removal and cooling as the steam is circulated by the circulation subsystem away from the heat transfer plate. In addition, the super-cooled liquid consisting of liquid ice maintains steam bubbles associated with melted liquid ice boiling small, resulting in more effective heat transfer. Finally, use of liquid ice as a refrigerant or coolant is compatible with cooling systems aboard ships operated by the U.S. Navy, thereby satisfying the apparent desirability and advantage to integrate the cooling system in accordance with the embodiments of the present invention with other cooling systems on a ship (for example, air conditioning systems).
The foregoing and other objects, features, and advantages of the present invention will become more readily apparent from the following detailed description of various embodiments, which:proceeds with reference to the accompanying drawing.